In recent years, there have been proposed and developed various fail-safe technologies for variable valve timing control systems. One such fail-safe technology has been disclosed in Japanese Patent Provisional Publication No. 5-98916 (hereinafter is referred to as “JP5-98916”).
In the variable valve timing control system disclosed in JP5-98916, two variable valve timing control mechanisms are respectively arranged in two cylinder banks for a V-type internal combustion engine. When a failure or malfunction of the variable valve timing control mechanism arranged in a first bank of the two cylinder banks is detected, a desired valve timing of the variable valve timing control mechanism arranged in the second bank is forcibly adjusted or brought closer to an actual valve timing of the variable valve timing control mechanism that is arranged in the first bank and fails to function properly. This effectively avoids valve timings of the two cylinder banks from undesirably fluctuating and unbalancing to each other, even in presence of a failure in the variable valve timing control mechanism or a valve timing control system failure, and thus prevents an extremely unstable state of the engine from occurring.
Later automotive vehicles often employ a variable valve lift and working angle control mechanism as well as a variable valve timing control mechanism. Generally, there are two types of variable valve lift and working angle control mechanisms, namely, one being a high-speed cam/low-speed cam switching system in which a valve lift and a working angle are both variable by switching between a high-speed cam enabling a large working angle and a large valve lift and a low-speed cam enabling a small working angle and a small valve lift, and the other being a so-called continuous variable valve event and lift control system, often abbreviated to “VEL”, in which a valve lift and a working angle are both continuously simultaneously variably controlled.
When a plurality of variable valve timing control mechanisms arranged in respective cylinder banks of a multi-cylinder-bank engine and a variable valve lift and working angle control mechanism common to the cylinder banks are combined with each other, it is possible to increase a degree of freedom of setting of valve lift characteristics of engine valves (intake and exhaust valves), thus ensuring improved fuel economy, that is, reduced fuel consumption and enhanced engine performance such as increased engine power output and enhanced combustion stability. The avoidance of degraded engine performance (a drop in engine output torque), which may occur owing to unbalanced valve timings, would be desirable even in presence of a failure in a certain variable valve timing control mechanism or a malfunction in a variable valve timing control system, on internal combustion engines equipped with a plurality of variable valve timing control mechanisms and a variable valve lift and working angle control mechanism combined with each other.